1. Field of the Invention
The present invention relates to golf balls. More particularly, the present invention relates to improvement of covers of golf balls.
2. Description of the Related Art
Golf balls hit by a golf club make a flight with accompanying back spin. This back spin results from shearing force which occurs upon impact of the golf ball on a head having a loft angle. Lift force acts on the golf ball through the back spin, leading to optimization of the height of a trajectory. Spin speed largely influences on the height of a trajectory and flight distance.
A golf ball that fell on the green rolls on the green and stops in due course. The movement from the falling point to the stop point is referred to as run or roll. Larger run will result in tumbling of the golf ball out of the green, or in long distanced stop point from a cup, which may lead to difficulties in patting thereafter. In instances of shots aiming at the green (in many instances, shots by an iron golf club), golf balls which provide less run are preferred. The faster the rotation speed of back spin upon falling becomes, the golf ball becomes more apt to stop running on the green. This is caused by rotational direction of the back spin which is reverse to rotational direction of the rolling golf ball.
A golf ball after impact may often make a flight with accompanying side spin. Side spin in a right direction results in a fade ball, and side spin in a left direction results in a draw ball. Golfers occasionally hit the fade or draw intentionally. On behalf of golf balls which are apt to be accompanied by side spin, golfers can easily hit the fade or draw.
Spin performances are thus significantly important to golf balls. General golf balls have a core and a cover. Because the cover is positioned on an outermost side except for a paint layer, it greatly participates in mechanisms of spin occurrence. A variety of improvements of cover materials have been proposed in an attempt to enhance spin performances (for example, Japanese Patent Laid-open No. 305115/1998).
Because spin performances are important demand characteristics, golfers have asked for further enhancement of spin performances. In recent years, golf balls with a cover having small thickness have been developed. According to such a type of golf balls, contribution ratio of the cover to spin performances tends to be small. Therefore, there exist urgent needs for improvement of qualities of materials for such a thin cover.